The vertebrate skeleton is an important anatomical system providing structural support for a motile body, protection for vital internal organs, and a hospitable environment for the hematopoetic system. The set of processes by which these bony structures develop is known as skeletal morphogenesis. Defects in skeletal morphogenesis are manifested as osteochondrodysplasias, a large group of statural disorders affecting more than 1 person in 5000 in the human population and observed in nearly one percent of perinatal deaths. These conditions range in seriousness from relatively mild to severe. The more mild conditions may affect such quality of life issues as stature, mobility, activity and sexuality. Moderate conditions may include neurological and orthopedic complications. The most severe cases can result in perinatal lethality. The purpose of this proposal is to study the molecular genetic basis of disproportionate dwarfism seen in mice carrying the rhizomelia (rzm) mutation, rzm is a novel, transgene-induced mutation that causes skeletal growth aberrations in both the axial and appendicular skeleton including shortened humeri and femora, narrowed thoracic cavity, shortened tail, and craniofacial defects. The specific aims of this proposal are focused at understanding the molecular genetic basis of the rzm phenotype. First, experiments will be performed to determine whether the rzm phenotype results from a gain-of-function (GoF) or loss-of-function (LoF) mechanism. Using an embryonic stem (ES) cell-based approach, a deletion (delta) will be constructed which spans the rzm locus. After creating deletion-bearing mice, genetic crosses will be used to place rzm in trans to the encompassing deletion (rzm/A). Assessment of the resulting phenotype should allow one to discriminate between the GoF and LoF hypotheses. Second, molecular studies will be performed to clone the genomic region flanking the transgene insertion and to assess the expression of linked candidate genes and other critical players implicated in skeletal morphogenesis. The ultimate goal of this and subsequent studies is to use this unique animal and the molecular tag provided by the transgene itself as a means of entry into the network of structural and regulatory genes that coordinate skeletal development.